Mass spectrometer



Aug. 9, 1960 A. J L. VAN EGMOND ETAL MASS SPECTROMEITER Filed Feb. 18, 1957 FIG.2

FIG.3

!NVENTOR ALBERT JEAN LOUIS VAN EGMOND PIETER CORNELS VEENSTRA MASS SPECTROMETER Albert Jean Louis Van Egmond and Pieter Cornelis Veenstra, Utrecht, Netherlands, assignors, by mesne assignments, to North American Philips Compauy, Inc., New York, N.Y., a corporation of Delaware Filed Feb. 18, 1957, Ser. N0. 640,790

Claims priority, application Netherlands Feb. 27, 1956 Claims. (Cl. 250-41.9)

The present invention relates to mass spectrometers for determining the ratio of the concentrations of two constitutents of a gas mixture.

As is well known, a mass spectrometer generally contains an ionization charnber in which the molecules of a gas to be examined are subjected to bombardment by an electron stream so that the gas molecules are ionized. Under the action of an electric field set up by the accelerating electrode or electrodes, the electrons produced are conveyed at a certain velocity to a space in which they are subjected to the action of a magnetic field, an electrc field, or both a magnetic and an electric field. Consequently, the ions travelling through this space are deflected -from their initially rectilinear path, the degree of deflection depending upon the mass-charge ratio of each ion. In the field, the paths of the ions become substantially circular and separate ion beams are formed each consisting of ions of the same constituent of the initial gas mixture. electrodes or the deflecting field, each beam can be focussed onto the collector electrode. The strength of the current to the collector is a measure of the concentration of the constituent of the mixture trom which the ions concerned orignate. Generally, the collector current is very weak and must be amplified before it can be measured.

In a known mass spectrorneter, an image of the current to the collector is produced on the sereen of a cathoderay oscillograph as a function of the accelerating voltage. The curve produced exhibits a number of maximum values corresponding to the components of the gas mixture, so called mass peaks, the heights of which are a measure of the concentrations of these components. A comparison between the heights of two mass peaks indicates the ratio of the concentrations of the corresponding two constituents of the mixture.

A device is known in which the aforementioned ratio is determined by impartng to the voltage at the accelerating system in cyclic sequence the values at which the ions of each constituent of the mixture to be examined are focussed onto the collector electrode. This is erected by means of a rotating step switch which successively actuates a number of relays through which the acceler ating voltages (which are taken trom potentiometers) are supplied to the accelerating electrodes of the mass spectrometer. The output circuit of the amplifier for the collector currents is connected to a number of adjustable atenuators, for example potentiometers. The output voltages of attenuators are successvely supplied, through further relay contacts, in synchronism with the variation of the accelerating voltages, to a comparison device. The attenuators are adjusted so that in the comparison device the voltage peaks corresponding to the various masses are equal to one another. Thus, the adjustment of the attenuators determines the concentrations of the con stituents of the mixture. This device has the advantage that the measurements are only slightly aflected by acci- By varying the voltage at the accelerating atent O icc dental variations of the sensitivities of the mass spectrometer and the amplifier.

The present invention relates to a mass spectrometer of this kind for determining the ratio of the concentrations of two constituents of a gas mixture to be analyzed. It is an object of the inventon to provide a comparatively simple device by which this ratio can be determined rapidly and accurately.

In accordance with the present invention, shortly be tore each periodic change of the accelerating voltage or, in general, of the quantities which determine the movement of the various ions, the ion flow is interrupted. After a certain period of time the ion flow is restored again, so that an interrupted sequence of square waves is produced in synchronism with the periodic changes of the accelerating voltage. Two capacitors are charged to the mean voltages of the alternating voltage waves which correspond to the constituents of the mixture.

The peak detection of the arrangement of the invention results in independence of the switching times of the switches. The switches may consequently be simple bipolar relays fed from the alternating current supply. The periodic interruption of the ion stream ensures that no mass peaks can occur which correspond to constituents of the mixture whose mass peaks lie between those of the constituents whose concentration ratio is to be ascertained.

A switch, which is in synchronism with the interruption of the ionization, for part of the duration of the voltage wave corresponding to one of the constituents of the gas mixture, completes the output circuit and supplies the voltage waves which are passed to a double-pole switch which operates at half the frequency of the first-mentioned switch and through which two capacitors are alternately connected to the output circuit and charged.

The diierences of the voltages across the capacitors can be measured by means of a direct-current instrument. The attenuator producing the output voltage which corresponds to the gas having the highest mass peak is adjusted so that the measuring instrument indicates zero. In this arrangement, only a single attenuator is required; the other voltage can be passed on unattenuated. The measuring instrument preferably has its zero indication at the center of its scale.

When using the alternating current supply as the volt age supply source of the relays, 25 measurements per second are performed for each of the constituents of the mixture, so that the influence of accidental variations of the ion source and the amplfier used are eliminated.

The attenuator can be adjusted automatically by means of a servomechanism. In this event, the voltage difference if required after amplification, acts upon a motor driving the operating mechanism of the attenuator.

The voltage ditference can be used for an automatic control of a composition of the gas mixture, in which event the voltage dilference is used as the control voltage of the controller.

The ratio of the concentratons of the two constituents is determined by the adjustment of the attenuator.

In order that the invention may readily be carried intoeiect, it will now be described, by way of example, with reference to the acornpanying drawing, in which:

Fig. 1 is a schematic diagram of an embodiment of the circuit arrangement of the present invention;

Fig. 2 is a graphical presentation of various voltages produced at various points of the embodiment of Fig. 1 as functions of the time, indicating the switching instants of various relays of the embodiment of Fig. 1; and

Fig. 3 is a schematic diagram of an embodiment of a circuit arrangement which may be utilized in the em bodiment of Fig. 1.

In Fig. 1, a spectrometer tube 1 of a type known in Patented Aug. 9, 1960 the art, whose design ancloperating characteristics are assumel to beknown, comprises a grid 12 by means of which the electron current ionizing the gas mixture can be controlled, an electrode.l3 by means of which the ions an c er te q srsat =r ss e t and a collector electrode 14. It is assurned that the ion; are acted upon by a magnetic field in a marmer whereby they describe substantially circular pat hs in the field.

The collector currents low through a high resista nce 5 and the voltages, produced acros s said resistor a re arnplified in an amplifier 2. The amplifier 2 ensures that despite the high input resistance and input capacitance a satisfactory frequency characteristic is obtained so that the. square-wave input voltages are transmitted as square wave output voltages.

By means of a switch a, a voltage of the form shown in Fig. 2ais applied to the ionization electrode 12.

The switch a can be designedas a bipolar relay which is fed from the source of power supply. Thus, the ion source is alternately interrupted and restored at a frequency of 50 cycles per second so that each time during 0.01 second ions are produced and during the next 0.01 second there is no ion ization. Obviously, other means may be used to suppress the ion beam, such as for example, a highly deflecting auxilary field set up in, or in the vicinity of, the ion source.

An accelerating voltage, which is a square wave voltage having a frequency of 25 cycles per second, is applied to the accelerating electrode 13. The variation of the voltage applied to the electrode 13 is shown in Fig 26. The value of the accelerating voltage is raised from V to V+AV at the middle of the interval that no ions ar e produced at an interval of 0.02- second later, that is, at an instant that no ions are produced, the voltage is again given the value V. The value V corresponds to the mass peak of one constituent and the value V+AV corresponds to the mass pealg of the second constituent of the mixture.

At the collector, a current appears of the form shown in Fig. 20. The voltage at the point A in the output circuit of the amplifier 2 varies in the same mannen High and low voltage waves, which correspond to the various concentrations in the gas mixture, succeed one another.

The amplified voltage is set up across a potentiorneter 6. By means of a switch d ether the point A or the sliding contact of the potentiometer 6 is periodically connected to the point B. Consequently, the highest square wave is attenuated and the voltage at point B has the form shown in Fig. 2e. The manner in which the switch d operates is shown in Fig. 2d. The movable arm of the switch alternately engages the point A fixed con tact and the potentiorneter 6 sliding contact for 0.02 second. Thus the switch d operates at a frequency of 25 cycles per second. Fig. 2e shows the condition in which equality of the pulses has not yet been achieved. As will be shown hereinafter, thesliding contact of the potentiometer 6 is.invariahly arranged so as to achieve equality.

The voltage of the form shown in Fig. 2eis transmitted by an amplifier 3, which generally comprises only a single cathode follower, to. adevice for comparing the two voltages. The comparison device comprises a switch 2, a switch and two capactors 7 and 8. The switch e operates in the marmer shown in Fig. 2) and periodically establishes a direct connection to the movable contact of the switch f for a certain period of time. The contact e is closed shortly after each appearance of a voltage and is opened shortly before it disappears. Th is may be etected by a simple relay with delayed operation.

Through the switch f, the capacitors 7 and 8 are alternately connected to one of the contacts of e. This is eiected in the manner shown in Fig. 2g at a.frequency '4 of 25 cycles per second and in synchronism with the va.riation of the accelerating voltage.

The capacitors 7 and 8 are charged by the voltages corresponding with the square-wave voltages shown in Fig. 2e. These voltages are shown in Fig. 2h. Charging may be performed through aresistor R in the output circuit of the amplifier 3 and this ensures that the capacitors 7 and 8 are charged to voltages which have substantially the mean value of the voltage of each pulse. The resistor R may be of the order of 10,000 ohms and the capacitance values of the capacitors 7 and 8 may be of the order of 1 microfarad.

During the periods in which no voltages are set up at the point A due to ions, the mean potential of said point should be zero. This may be achieved by means of a switch c which is shown in Fig. 1 and is closed in the intervals in which there is no votlage. In this event, the amplfier 2 must be coupled to the potentiometer 6 through the capacitor 30. However, this involves an increase of the noise. Noise of frequencies of 2511 cycles per second (where n is an integer) provides a considerable additional contribution to noise of 25 cycles per second, the frequency band in which measurements are actually carried out. The influence of noise voltages can be eliminated in the manner shown in Fig. 3. The circuit arrangement shown in Fig. 3 also acts as an automatic correction circuit for the zero leve]. No coupling capacitor is required.

In the circuit arrangement of Fig. 3, provision is made of a tube 15 which is connected as a cathode follower. The control grid of the tube 15 is connected to the output of the amplifier 2 and the cathode of said tube, which is connected to a point at negative potential through a high resistance, is also connected to the point A. The junction of two resistors 17 and 17' is connected to an amplifier 18 which is provided with a filter tuned to 50 cycles per second. The filter ensures that the amplifier 18 is not overdriven by its own noise. The output of the filter is connected, through a switch k to an RC circuit 21, 22. By means of a tube 23, the output voltages of the RC circuit 21, 22 are amplified and supplied to the grid of the tube 15. The switch h, which bridges the resistor 17, and the switch k operate in the manner indicated in Fig. 2k; the switch h is closed during a pulse and is open for the larger part of the intervals, as is indicated by cross-hatching in Fig. 2k.

When the circuit arrangement shown in Fig. 3 is used, there is no ncrease in the noise, for care is taken that the ampliication by the amplifier 2 multiplied by the attenuation in the RC-filter is less than 1.

Fig. 1 shows another amplifier 4, to the output of which is connected a switch g by means of which the amplified voltage pulses are alternately suppled to capacitor 9 or capacitor 10. A direct current instrument 11, for example a tube voltage measuring circuit, indicates the potential difference between the capacitors. In measuring, the sliding contact of the potentiometer 6 is moved until the instrument 11 indicates zero. The movement of the potentiometer contact may be effectecl by means of a servomechanism. This arrangement together with the switches 1 and g measures the voltage between the capacitors 7 and 8 through a frequency and phase-selective amplifier with a low noise leve]. The amplifier is a conventional alternating voltage amplfier.

The relays operating at 50 cycles per second may be esigned as polarized relays and may be directly connected to the source of power supply. The relays operating at 25 cycles per second may also be controlled from the alternating current supply. The 25 cycle per second relays are preferably designed as electronic frequency dividers, for example flip-flop circuits combined with phase shifters by means of which squarewave voltages having a recurrence frequency of 25 cycles per second are produced, the phases of which can be adjusted at will.

The circuit arrangement of the present inventiou per mits the direct measurement of isotope ratis, even if the masses difier from one another by more than a mass unit.

While the invention has been described by means of a specific example and in specific embodiments, we do not wish to be lirnted thereto, for obvious modifications will occur to these skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A mass spectrometer circuit arrangement comprising a spectrometer tube having means for controlling the ionization of a gas mixture in said tube, means for controlling the acceleration of the ions of said gas mixture and means for collecting said ions, switching means for cyclically permitting the ionization of said gas mixture, means for cyclically applying acceleration voltages having difierent levels of magnitude during consecutive periods to said acceleration controlling meansthereby producng an output voltage at said collecting means comprising a series of pulses having consecutive levels of magnitude correspondng to the ions produced by the oom ponents of said gas mixture, said switching means and said means for cyclically applying acceleration voltages having a predetermined frequency and phase relationship to each other, means for detecting said output voltage in synchronisrn with the operation of said switching means to produce at least two voltages having magnitude levels determined by the ions produced by the components of said gas mixture, and means for determining the difference between said two voltages thereby establishing the ratio of the concentrations of ions of the components of said gas mixture.

2. A mass spectrometer circuit arrangement comprising a spectrometer tube having means for controlling the ionization of a gas mixture in said tube, means for controlling the acceleration of the ions of said gas mixture and means for collecting said ions, switching means for cyclically permitting the ionization of said gas mixture, means for cyclically applying acceleration voltages having diterent levels of magnitude during consecutive pe riods to said acceleration controlling mean thereby producing an output voltage at said collecting means comprising a series of pulses having consecutive levels of magnitude corresponding to the ions produced by the components of said gas mixture, said switching means and said means for cyclically applying acceleration voltages having a predetermined frequency and phase rela tionship to each other, means for detecting said output Voltage in synchronism with the operation of said switching means to produce at least two voltages having magnitude levels determined by the ions produced by the components of said gas mixture, said detecting means comprising at least a pair of capacitors and means for applying said output voltage to said capacitors in synchronism with the operation of said switching means, and means for determining the difference between said two voltages thereby establishing the ratio of the concentrations of ions of the components of said gas mixture.

3. A mass spectrometer circuit arrangement comprising a spectrometer tube having means for controlling the ionization of a gas mixture in said tube, means for controlling the acceleration of the ions of said gas mixture and means for collecting said ions, switching means for cyclically permitting the ionization of said gas mixture, means for cyclically applying acceleration voltages having difierent levels of magnitude during consecutive periods to said acceleration controlling means thereby producing an output voltage at said collecting means comprising a series of pulses having consecutive levels of magnitude correspouding to the ions produced by the components of said gas mixture, said switching means and said means for cyclically applying acceleration voltages having a predetermined frequency and phase relationship to each other, means for detecting said output voltage in synchronism with the operation of said switching means to produce at least two voltages having magnitude levels determined by the ions produced by the components of said gas mixture, said detecting means comprising at least a pair of capacitors, means for applying said output voltage to said capacitors in synchronism with the operation of said switching means and means for alteruately charging said capacitors proportionally to the levels of magnitude of said output voltage in sy'nchronism with the operation of said switching means, and means for determim'ng the diierence between said two voltages thereby establishing the ratio of the concentrations of ions of the compouents of said gas mixture.

4. A mass spectrometer circuit arrangement comprising a spectrometer tube having means for controlling the ionization of a gas mixture in said tube, means for controlling the acceleration of the ions of said gas mixture and means for collecting said ions, switching means for cyclically permitting the ionization of said gas mixture, means for cyclically applying acceleration voltages having different levels of magnitude during consecutive periods to said acceleration controlling means thereby producing an output voltage at said collecting means comprising a series of: pulses having consecutive levels of magnitude corresponding to the ions produced by the components of said gasmixture, said switching means and said means for cyclically applying acceleration voltages having a predetermined frequency and phase relationship to each other, means for detectiug said output voltage comprisinga pair of capacitors and means for applying said output voltage to said capacitors in synchronism with the operation of said switching means whereby each of said capacitors is alternately charged to produce a voltage having a magnitude level determined by the ions produced by a component of. said gas mixture, and means for determining the difierence between the voltages =across said capacitors thereby establishing the ratio of the concentrations of ions of the components of said gas mixture.

5. A mass spectrometer circuit arrangement comprising a spectrometer tube having means for controlling the ionization of a gas mixture in said tube, means for controlling the acceleration of the ions of said gas mixture and means for collecting said ions, switching means for cyclically perrntting the ionization of said gas mixture, means for cyclically applying acceleration voltages having difierent levels of magnitude during consecutive periods to said acceleration controlling means thereby producing an output voltage at said collecting means comprising a series of pulses having consecutive levels of magnitude corresponding to the ions produced by the components of said gas mixture, said switching means and said means for cyclically applying acceleration voltages having a predetermined frequency and phase relationship to each other, means for detecting said output voltage compri sing a pair of capacitors, a resistor and means for applying said output voltage to said capacitors through said resistor in synchronism with the operaton of said switching means whereby each of said capacitors is alternately charged proportionally to the mean value of a magnitude level of said output voltage to produce a voltage having a magnitude level corresponding to the concentration of ions produced by a component of said gas mixture, and means for determining the ditference between the voltagesacross said capacitors thereby establishing the ratio of the concentrations of ions of the components of said gas mixture.

6. A mass spectrometer circuit arrangement compris ing a spectrometer tube having means for controlling the ionization of a gas mixture in said tube, means for controlling the acceleration of the ions of said gas mixture and means for collecting said ions, switch means for cyclically permittingthe ionization of said gas mixture, means for cyclically applying acceleration voltages having dierent levels of magnitude during consecutive periods to sad acceleration controlling means thereby producing anoutput voltage at sad collecting means comprising a series of pulses having consecutive levels of magnitude corresponding to the ions produced by the components of sad gas mixture, sad switching means and sad means for cyclically applying acceleration voltages having a predeterrninecl frequency and phase relationship to each other, means for detecting sad output voltage comprising, a pair of capacitors, second switchng means interposed between sad collecting means and sad capacitors for applying sad output voltage to sad capacitors in synchronism with the operation of sad firstmentioned switching means and third switching means in terposed between sad second switching means and sad capacitors for alternately applying the voltage passed by sad second switching means to each of sad capacitors vvhereby each of sad capacitors is alternately charged to produce a voltage having a magm'tude level determned by the ions produced by a component of sad gas mixture, and means for determining the difference oetween the voltages across sad capacitors thereby establishing the ratio of the concentrations.ofions of the components of sad gas-mixture.

7. A mass spectrometer circuit arrangement compris ing a spectrometer tube having means for controlling the ionizationof a gas mixture in sad tube, means for controlling the acceleration of the ions of sad gas mixture and means for collectiug sad ions, switchng means for cyclically permitting the ionizaton of sad gas mixture, means for cyclically applying acceleration voltages having dterent levels of magnitudeduring consecutive pe riods to sad acceleration controlling means thereby producing an output voltage at sad collecting meanscomprising a series of pulses having consecutive levels of magnitude correspondng to the ions produced by the components of sad gas mixture, sad switching means and sad means for cyclically applying acceleration voltages having a predetermined frequency and phase relationship to each other, means for detecting sad output voltage comprising a pair of capacitors, second switching means interposed between sad collecting means andsaid capacitors forapplying-said output voltage to sad capacitors in synchronism with the operation of sad firstmentioned switching means andthird switching means interposed between sad secondswitching means and sad capacitors for alternately applying the voltage passed by sad second switching means to each of sad capacitorsat half the -frequeucy of operation of sad second switch ing means whereby each of saidcapacitors is alternately charged to produce a voltage having a magnitude level determined by the ions produced by a component of sad gas mixture, and means for deterrnirring the diierence between the voltages across sad capacitors thereby estab lishing the ratio of the conceutrations of ions of the components of sad gas mixture.

8. A mass spectrometer circuit arrangement comprising a spectrometer tube having means for controlling the ionization of a gas mixture in sad tube, means for controlling the acceleration of the ions of sad gas mixture and means for collecting sad ions, switchir1g means for cyclically permittng the ionzation of sad gas mixture, means for cyclically applying acceleration voltages havingdilerent levels of magnitude during consecutive periodsto sad acceleration controlling means thereby producng an output "oltage at sad collecting means comprising a series of pulses having consecutive levels of magnitude corresponding to the ions produced by the components of sad gas mixture, sad switching means and sad means for cyclically applying acceleration voltages having a predetermined frequency and phase relationship to each other, means for detectng sad output voltage comprising a pair of capacitors, second switching means interposed between sad collecting means and sad capacitors for applying sad output voltage te sad capacitors in synchronism with the operation ofsaid firstmentioned switching means, sad second switehing means applying sad output voltage to sad capacitors for a pcriod of time beginning relatively soor1 after the initation of operation of sad first-mentionedswitching means and ending relatively shortly before the cessation of operatiot1 of the sad first-mentioned switchng means, and third switching means interposed between sad second switching means and sad capacitors for alternately applying the voltage passed by sad second switching means to each of sad capacitors at half the frequency of operation of sad second switching means whereby each of sad capacitors is alternately charged to produce a voltage having a magnitude level determined by the ions pro duced by a component of sad gas mixture, and means for determining thedifierence between the voltages across sad capacitors thereby establishing the ratio of the con centrations of ions of the components of sad gas mixture 9. A mass spectrometer circuit arrangement compris ing a spectrometer tube having means for controlling the ionization of a,gas mixture in sad tube, means for controlling the acceleration of the ions of sad gas mixture and means for collecting sad ions, switching means for cyclically permittiug the ionizatiou of sad gas mixture, means for cyclically applying acceleration voltages having difierent levels of magnitude during consecutive periods to sad acceleration controlling means thereby producing an output voltage at sad collecting means comprising a series of pulses having consecutive levels of magnitude corresponding to the ions produced by the components of sad gas mixture, sad switching means and sad means for cyclically applying acceleration volt ages having a predeterminedfrequency and phase relationship to each other, means for detecting sad output voltage comprisiug a pair of capacitors, second switching means interposed between sad collecting means and sad capacitors for applying sad output voltage to sad capacitors in synchronism with the operation of sad firstrnentioned switching means, sad second switching means applying sad output voltage to sad capacitors for a period of time beginning relatively soon after the initiation of operation of sad first-mentioned switching means and ending relatively shortly before the cessation of operation of the sad first-mentioned switching means, and third swtching means interposed between sad second switching means and sad capacitors for alternately apply ing the voltage passed by sad second switching means to each of sad capacitors at half the frequency of operation of sad second switching means whereby each of sad capacitors is altemately charged te produce a voltage having a magnitude level determined by the ions produced by a component of sad gas mixture, sad third switchng means comprising a tWo-position switch, and means for determining the difference between the voltages across sad capacitors thereby establishng the ratio of the concentrations of ions of the components of sad gas mixture.

10. A mass spectrometer circuit arrangement comprzsing a spectrometer tube having means for controlling the onization of a gas mixture in sad tube, means for controlling the acceleration of the ions of sad gas mixture and means for collecting sad ions, swtching means for cyclically permitting the ionzation of sad gas mixture, means for cyclically applying acceleration voltages having difierent levels of magnitude during consecutive periods to sad acceleration controlling means thereby producing an output voltage at sad collecting means compris ing a series of pulses having consecutive levels of magnitude corresponding to the ions produced by the components of sad gas mixture, sad switching means and sad means for cyclically applying acceleration voltages having a predeterrnined frequency and phase relation 9 shp to each other, means for detecting sad output voltage comprising a pair of capactors, second switchng means interposed between sad collecting means and sad capactors for applying sad output voltage to sad ca pactors in synchronsm With the operaton of sad firstmentioned switchng means, sad second swtchng means applying sad output voltage to sad capacitors for a perod of time begnning relatively soon after the initiation of operaton of sad first-mentioned switching means and ending relatively shortly before the cessation of operati0n of the sad first-mentioned switchng means, and thrd switchng means interposed between sad second switching means -and sad capactors for alternately applying the voltage passed by sad second swtehing means tc eaoh of sad capacitors at half the frequency of operaton of sad second switchng means whereby each of sad capacitors is alternately charged to produce a voltage having a magnitude level determined by the ions pro duced by a component of sad gas mixture, each of sad second and third swtching means comprisng a bipolar relay, and means for determinng the dflerence between the voltages across sad capactors thereby establishng the ratio of the concentrations of ions of the components of sad gas mixture.

No references cted. 

